SITE EFFECTS USING AMBIENT NOISE FREQUENCY SURVEY IN TEHRI GARHWAL REGION

Abstract

The Tehri – Garhwal region of Uttarakhand state is of sheer importance primarily in the studies of Seismotectonics. High seismicity, elaborate and dynamic thrust system and Himalayan context have rewarded it with utmost interests of Seismologist and Geologists. Many seismic properties of site such as soft soil thickness, porosity, saturation of rocks, and quality factor and sub surface velocity of seismic waves are among many factors which influence the nature of devastation that can occur in case of an Earthquake. Hence, detailed evaluation of these parameters across the region will help in micro-zonation and designing structures. This thesis deals with the evaluation of sub surface velocity structure for 9 stations situated along a line from Rishikesh and Tehri dam. For a long time, Probabilistic Seismic Hazard Assessment (PSHA) has been the most important geophysical contribution in the field of seismic risk evaluation. But, in the recent times, mapping of site amplification effects has evolved itself as another significant factor. Several studies have already shown convergence of results based on these two elements. Site amplification requires knowledge of sub surface sheer wave velocity structure. These are then used to classify sub surficial elements into various regions of alike compaction. Numerous active and passive source methods are available for this purpose. But, due to its relatively simple approach, low cost and open data processing software, most widely used technique is the horizontal to vertical spectral ratio (HVSR) method. It is done using single station ambient noise records. This technique is based on idea propagated by Nakamura (1999) that peak of QTS (Quasi Transfer Spectrum) i.e. H/V spectral ratio plots are consequences of multiple reflections of SH waves. Sub surface velocity structure can be developed by inversion of this plot into parameters defining propagation of SH wave in horizontal layered medium. Here, Levenberg – Marquardt damped least square inversion using acceptable constraints is adopted for this purpose. Field work was carried out for collecting ambient noise data along 9 stations along Rishikesh Tehri road. Thesis includes 1D velocity structure for each stations and subsequent 2D sub surface velocity profile for line connecting these stations. It also talks about limitations and challenges faced during ambient noise frequency survey and its analysis. Finally, it talks about the relevance of this study in regional and spatial context.